The present invention relates to the control of electric motors, and in particular to the control of harmonics in the voltages applied to the windings of electric motors. It can be used, for example, to correct harmonic errors in the voltages output by motor drivers, and to add harmonics to the voltages input to the motor to correct harmonic variations in the torque output of the motor.
A closed loop current controller for an electric actuator typically takes the form shown in FIG. 1. Using vector control in response to a requested target current, a voltage is applied to each phase winding of the motor and the resultant generated currents measured. The measured currents are then used by a controller, typically a PI controller, to control to the requested voltage to achieve the target current. A signal which gives the electrical position of the motor is also required so that the voltages can be applied at the correct phases at the correct times. In the system of FIG. 1 a position sensor is shown but the present invention is equally applicable to a position sensorless system, where the position signal is estimated from other sensor measurements.
Typically for a 3 phase motor the driver comprises a 6 FET bridge arranged to apply a known voltage to the motor terminals in a PWM pattern to achieve the desired closed loop current. Due to the PWM switching patterns that are applied to the motor to produce the desired voltage, drive stage non-linearities in the form of additional unwanted harmonic frequencies can be introduced as shown in FIG. 2.